The present invention provides a plasma treatment system, and more particularly to a plasma treatment system for etching a semiconductor substrate and for plasma-treating a semiconductor thin film in manufacturing processes for electric devices and semiconductor devices.
The plasma treating system has a vacuum reaction chamber kept in a low gas pressure. A microwave is introduced into the reaction chamber to cause a gas discharge for causing a plasma which is to be irradiated onto a sample "S" such as a semiconductor substrate for etching the semiconductor substrate and for growing a semiconductor thin film. For drying etching technique and growth of a semiconductor thin film, it is required to control both the microwave for generation of the plasma and a power for acceleration of ions in plasma generated independently.
FIG. 1 is a cross sectional elevation view illustrative of a conventional plasma treatment system which is capable of controlling the generation of plasma and acceleration of ions in the plasma independently. The conventional plasma treatment system has a reaction chamber 1 which is rectangular-shaped and made of a metal such as aluminum and stainless steel. In the reaction chamber 1, a sample stage 8 is provided which has a sample holder 7 which holds a sample to be treated with plasma. A high frequency power 9 is provided which is electrically connected to the sample holder 7 for applying a high frequency voltage to the sample S. The sample stage 8 is provided on a bottom wall of the reaction chamber 1. Side walls of the reaction chamber 1 are formed thickly so that electrical heaters 15 are provided within the side walls of the reaction chamber 1. The heaters 15 are electrically connected to a heater power 19 for supplying a power to the heaters 15 so that the heaters 15 heat up inner space of the reaction chamber 1. Sheet type rubber heaters 16 is also provided on the bottom of the opposite end portions of the opposite electrode 11 for heating up the opposite electrode 11 for preventing deposition from adhesion on the opposite electrode 11. The sheet type rubber heater 16 is also connected to the heater power supply 19. A gas supply tube 13 is provided on a side wall of the reaction chamber 1 for supplying a reaction gas into the reaction chamber 1 so that the microwave is irradiated onto the a reaction gas introduced to cause a plasma. A discharge tube 12 is also provided on the side wall of the reaction chamber 1 for discharging the used gas from the reaction chamber 1. A dielectric film is provided on the top of the reaction chamber 1 which is made of a microwave-permissible dielectric such as quartz glass or Al.sub.2 O.sub.3 having a small dielectric loss and a high heat resistivity. The sample holder 7 serves as a cathode to be supplied with the high frequency power from the high frequency power supply 9. On the bottom surface of the dielectric film 2, an opposite electrode 11 serving as an anode and being made of aluminum is provided which has a plurality of windows through which microwaves are transmitted into the reaction chamber 1. The opposite electrode is also contacted with the side wall of the reaction chamber 1 and the side wall of the reaction chamber 1 is grounded by a ground line 10 so as to allow a strain free electric field to be generated between the electrodes and allow a uniform bias voltage on the sample S. A dielectric line 3 is provided which extends to cover the dielectric film 2 and to be spaced apart from the dielectric film 2. The dielectric line 3 comprises a dielectric layer made of a dielectric having a small dielectric loss such as fluorine resin. A metal plate 4 made of a metal such as aluminum is provided which laminates on the top of the dielectric line 3. The metal plate 4 extends not only over the top surface of the dielectric line 3 but also on the end of the dielectric line 3 for sealing the end of the dielectric line 3. A waveguide 6 is provided which is connected with the dielectric line 3 for guiding the transmission or propagation of the microwave. A microwave oscillator 5 is provided which is connected to the waveguide 6 for generating the microwave which is to be transmitted or propagated through the waveguide 6 and the dielectric line 3 and also through the microwave-permissible dielectric plate 2 and the windows 14 into the inner space of the reaction chamber 1 so that the microwave is irradiated onto the reaction gas introduced to cause the plasma which is to be irradiated onto the sample for plasma treatment.
The above plasma treatment system is operated as follows. The used gas us discharged through the discharge tube 12 to reduce the pressure of the gas in the reaction chamber 1 at a predetermined value. Thereafter, the fresh reaction gas is introduced through the gas supply tube 13 into the reaction chamber 1. A microwave is generated by the microwave generator 5. The generated microwave is then introduced through the waveguide 6 into the dielectric line 3 whereby an electric field is generated in the space positioned under the dielectric line 3. The generated electric field is transmitted through the microwave-permissible dielectric plate 2 and the windows 14 of the opposite electrode 11 into the inner space of the reaction chamber 1. Since the reaction gas has been introduced into the inner space, the electric field or the microwave is applied to the reaction gas whereby a plasma is caused. On the other hand, the high frequency voltage is applied to the sample holder 7 holding the sample S for causing a bias voltage on the surface of the sample S so that the bias voltage controls energy of ions in the plasma generated. The bias voltage causes the ions of the plasma to be irradiated onto the top surface of the sample S in the right angle to the top surface of the sample S for plasma treatment of the sample S.
FIG. 2 is a plane view illustrative of the opposite electrode 11 having stripe shaped windows 14. The sheet type rubber heater 16 is provided on the peripheral portion of the opposite electrode 11. The opposite electrode 11 made of aluminum is exposed to the plasma generated in the inner space of the reaction chamber 1. The opposite electrode is made of an aluminum-based material and is coated with alumite. The exposure of the opposite electrode made of the aluminum-based material causes a clack on the alumite coating layer on the opposite electrode whereby aluminum of the opposite electrode reacts with fluorine in the reaction gas to form AlF.sub.3 which is dust particles.
As examination, 25 dummy wafers have been treated under conditions of CF.sub.4 /CH.sub.2 F.sub.2 =40/40 sccm,.mu./RF=1300/600 W and 2 minutes of exposure of the plasma. Immediately thereafter, the dummy wafers are carried in the reaction chamber 1 during which the reaction gas is blown under the condition of CF.sub.4 /CH.sub.2 F.sub.2 =40/40 sccm without generation of plasma. About 20000 dust particles of 0.4 micrometers in diameter have been observed on the wafer of 8 inches in diameter. The dust particles were analyzed by an X-ray photo-electro spectroscopic analyzer to determine the dust particles to be AlF.sub.3. It was confirmed that the provision of the opposite electrode 11 made of aluminum-based material in the reaction chamber 1 to expose the opposite electrode 11 to the plasma results in generation of many dust particles of AlF.sub.3.
The dielectric plate 2 is made of the quartz or Al.sub.2 O.sub.3 and has a thickness of 2 cm. Since quartz glass and Al.sub.2 O.sub.3 have large heat capacities, the temperature of the dielectric plate 2 is increased up to the saturation temperature by the plasma treatment carried out in the reaction chamber 1. The increase in temperature of the dielectric plate 2 causes the deposition to release or discharge from the dielectric plate 2. This provides a great deal of influence to the etching properties, for example, the etching rate and selective ratio of etching are changed.
As examination, 7 dummy wafers of polysilicon have been treated under conditions of CF.sub.4 /CH.sub.2 F.sub.2 =40/40 sccm,.mu./RF=1300/600 W and 2 minutes of exposure of the plasma during which variations in etching rate of the polysilicon wafer and temperature of the center portion of the dielectric plate for every wafers have been obverted. The heat resistive dielectric plate 2 has been preheated by the heater 15 provided in the reaction chamber 1 and also by the sheet-type rubber heater 16 provided on the peripheral portion of the opposite electrode 6, wherein the heaters 15 and 16 have been set at a temperature of 170.degree. C. Notwithstanding, the center portion of the dielectric film 2 has a temperature of 143.degree. C. This means it difficult to realize a uniform heating up of the dielectric plate 2 by use of the rubber heater 16. The measured variations in etching rate of the individual polysilicon wafers and in temperature of the individual dielectric films are shown in Table 1.
TABLE 1 ______________________________________ Number of wafers 0 1 2 3 4 5 6 7 ______________________________________ Temperature 143 158 163 165 174 176 178 177 (.degree. C.) Etching-Rate 542 438 396 390 386 386 389 385 (A/min) ______________________________________
In the above circumstances, it had been required to provide a plasma treatment system for treating a semiconductor wafer or a semiconductor thin film with a plasma, which is capable of preventing generation of dust particles and of keeping a dielectric film provided in the system at a constant temperature without variation in temperature of a center portion of the dielectric film.